1. Field of the Invention
The present invention relates to a free piston shock tube/tunnel, and more particularly, to a free piston shock tube/tunnel with an improved piston stop having a means to control the movement of the piston during the last distance of its motion along the compression tube toward the diaphragm.
2. Description of the Prior Art
Free piston shock tube/tunnels have existed since the 1950's. During operation, free piston shock tube/tunnels are able to generate a shock wave of extremely high pressure and high temperature at a test site for a desired duration or test time. Free piston shock tube/tunnels are principally used to provide test conditions for aerodynamic conditions and studies relating to rocket nose cones, space re-entry vehicles, and other hypersonic aircraft.
In general, a free piston shock tube/tunnel includes an elongated, generally cylindrical compression tube containing a compression or driver gas such as helium. The compression tube is closed at one end by a diaphragm with a preselected rupture pressure. The compression tube is provided with a compression piston adapted for movement from a piston end of the tube toward the diaphragm end. Connected to the diaphragm end of the compression tube is an elongated shock tube having a test end remote from the diaphragm and being filled with a low pressure driven gas such as ambient air. When the piston is moved from the piston starting end of the compression tube toward the diaphragm end, the gas within the compression tube is compressed, thus generating pressure and causing the diaphragm to rupture. The rupturing of the diaphragm causes a volume of the compression gas to pass through the ruptured diaphragm and into the connected shock tube to form a shock wave. The shock wave compresses the driven gas during movement through the shock tube, thereby creating the desired test conditions at the test site. In the case of a shock tunnel, the compressed gas is further processed through a nozzle at the final test site.
The piston in a conventional free piston shock tube/tunnel is driven by compressed gas introduced behind the piston. During the compression movement of the piston toward the diaphragm, the gas in the compression tube can be compressed to pressures a high as 2,000 atmospheres or greater. This in turn can generate a shock wave in the shock tube which can create test conditions in the driven gas with temperatures as high as 12,000K. and pressures as high as 3,000 atm.
The shock tube is generally cylindrical construction having a single, constant diametrical dimension less than that of the diametrical dimension of the compression tube. In typical free piston shock tunnel structures, the diameter of the compression tube is at least about three times greater than the diameter of the shock tube.
Despite the utilization of free piston shock tube/tunnels for nearly 40 years, and despite continuing studies for the purpose of more fully understanding the operation, and optimizing the performance, of free piston shock tube/tunnels, their general construction has not changed significantly. A typical free piston shock tunnel is disclosed in Patent Cooperation Treaty publication number WO 89/02071 by Raymond Stalker. Published studies relating to the performance and operation of free piston shock tube/tunnels include an article entitled "Pressure Losses In Free Piston Driven Shock Tubes" by N. W. Page and R. J. Stalker in Shock Tubes and Waves (14th International Symposium on Shock Tubes and Shock Waves), August, 1983 at page 118 and an article entitled "The Piston Motion In A Free Piston Driver For Shock Tubes And Tunnels" by Hans. G. Hornung at GALCIT, California Institute Of Technology, 1988.
One deficiency of prior art free piston shock tube/tunnels has been the lack of an efficient means for stopping or controlling the movement of the piston during its final stages of motion along the compression tube toward the diaphragm without causing damage to the piston or the shock tube. This deficiency and the problems which result become more prevalent as the size and speed of the piston increase. Several mechanisms currently exist in the prior art for stopping movement of the piston at the end of its stroke. In one such mechanism, the piston is stopped by a plurality of cylindrical pads of non-metallic material. Typically, four pads of fixed length have been used. However, due to the high temperatures and pressures developed in this area, such pads become charred. This results in the inside of the shock tube/tunnel being coated with an undesirable film of residue. After several operations, the pads are no longer useful. Further, only a limited range of conditions can be tested because of the fixed length of such pads. When conditions outside of this limited range have been tested, damage to the piston and/or the compression tube often results.
In a second mechanism, the piston is stopped by a simple decrease in compression tube diameter. This, however, has the disadvantage of either shortening the compression stroke of the piston or extending the length of the compression tube. In either event, the modification is a rigid structure which is not designed to accommodate off-design conditions. A mechanism of this type is suggested by J. W. Willard in a paper entitled "Design And Performance Of The JPL Free-Piston Shock Tube" presented at the Fifth Hypervelocity Techniques Symposium in March of 1967. This mechanism involves reducing the diameter of the compression tube at the diaphragm end by placement of a sleeve member within the compression tube. This sleeve member, however, is not designed to be compressed or collapsed upon engagement with the piston, has an outer diameter substantially the same as the internal diameter of the compression tube and is designed for a relatively low operational range of energies.
Thus, although various mechanisms exist for the purpose of stopping movement of the piston and minimizing possible damage to the piston and compression tube, problems continue to exist. Accordingly, there is a need in the art for an improved piston stop for a free piston shock tube/tunnel by which the impact of the piston can be minimized and in which the various other limitations of prior art mechanisms can be overcome.